Is there any way to alter a raincoat to make it slightly longer in the sleeve area? My gf says that the back presses against her whenever she extends her arms. She likes it a lot, but because of feeling slightly pressed, she doesn't wear it.

If you want to make a join in the material, there are types of glue that are specifically made for soft plastics.If you live in the u.k. Bostik make a glue like this, generally avaliable from small hardware shops rather than big chainstores. To make a sleeve longer you would have to get hold of some spare material from the manufacturer.